Roughly 50 Marines with the U.S. Fleet Anti-Terrorism Security Team (FAST) are on the ground in the Yemeni capitol of Saana, to defend the American diplomatic outpost that has been under siege by protestors.

Separate Marine Corps elements from the elite anti-terrorism team were deployed to Libya, in the wake of the deadly attack on the U.S. Consulate there.

"Although these security forces are equipped for combat, these movements have been undertaken solely for the purpose of protecting American citizens and property," according to the letter.

The FAST units will remain on the ground in both countries "until the security situation becomes such that they are no longer needed," the president added.

"These actions have been directed consistent with my responsibility to protect U.S. citizens both at home and abroad ... pursuant to my constitutional authority to conduct U.S. foreign relations and as Commander in Chief and Chief Executive," the letter states.

Earlier this year, House GOP lawmakers accused the Obama administration of not adhering to the War Powers Act when the White House ordered U.S. warships to the coast of Libya last March to support the NATO-led mission to oust former Libyan strongman Moammar Gadhafi.

"We have not exercised our constitutional responsibilities" to weigh in on matters of war, Rep. Walter Jones (R-N.C.) told Defense Secretary Leon Panetta in April during a House Armed Services Committee hearing on U.S. options in Syria.